1. Field of the Invention
The present invention relates to driver circuits, and in particular, to a digital driver circuit that provides a controlled slew rate at the output of the circuit.
2. Description of the Related Art
Driver circuits are used for communicating digital information between different portions of electronic systems. For example, in mixed signal systems that have both analog and digital circuits, information in the form of analog signals may be received by one section of an electronic system and converted into digital information. Once converted into digital form, the information may be communicated across the system to a microprocessor, microcontroller, or digital signal processor. Driving digital data between the different portions of an electronic system presents many design challenges. First, digital signal lines between devices in an electronic system usually have large capacitances and parasitic inductances associated with them. Large CMOS devices utilizing large current values are typically used to drive the large capacitive loads at high speeds. However, at high switching frequencies, the large currents through the parasitic inductances cause large voltage spikes that can exceed noise margin requirements. This switching noise becomes even more of a problem when the noise couples into the analog portions of a mixed signal integrated circuit.
Another challenge associated with designing digital drivers is to maintain a constant output current across variations in process, temperature, and supply voltage. The output current of a typical CMOS driver will vary with process, temperature, and supply voltage. In order to guarantee the desired operation at nominal values, typical drivers are designed to output a nominal current which is greater than the minimum required under normal conditions. However, in a fast process, this current can become several times more than the minimum required. This large current can result in high voltage slew rates which can inject noise into the analog portions of the mixed signal system.
Accordingly, it would be desirable to have a digital driver circuit with a controlled current output, or slew rate, to reduce the switching noise in the mixed signal system. Additionally, it would be desirable to have a digital driver that maintains a constant current output across process, temperature, and supply variations.